1. Field of the Invention
The present invention relates generally to hybrid fiber optic-coaxial cable TV networks, and more particularly, to the application of a spectral node splitting technique designed to reduce the number of cable TV subscribers sharing the same transmission frequency spectrum.
2. Discussion of the Related Art
The capability to provide cable TV subscribers with a greater number of revenue-generating electronic services has become increasingly important for commercial success in the entertainment as well as the telecommunications industries. The long distance telecommunication exchange companies, the local exchange companies, and local cable access television (CATV) companies, the satellite communications companies are all seeking the right combination of technologies to provide additional services to the their subscribers. The additional services potentially include video-on-demand, pay-per-view, interactive television and games, videoconferencing, video telephony, CATV, Internet access, online commerce, and telephone services. In order to provide any combination of the above-mentioned services in an economically viable manner, a distribution network of substantial capacity is required. Capacity, in this sense, refers primarily to the information carrying capability, which is substantially related to the transmission frequency spectrum bandwidth of the transmission medium.
A transmission medium having the capacity needed to provide the required services is optical fiber. Although it is expected that in some future point in time certain users may have optical fiber running into their homes or offices, under the present circumstances it is not economically feasible to deploy an all fiber network infrastructure at one time. Therefore, alternative network architectures are being conceived, considered, and selectively implemented continuously. Certain proposed architecture types indicate a general trend toward the deployment of fiber backbone architecture.
A network architecture, which is presently considered sufficiently economical and consequently is being implemented widely is a hybrid fiber optic-coaxial cable (HFC) network. In an HFC network feeder fibers run from a head-end to an HFC distribution node remotely located in respect to the head-end. At the HFC distribution node, the fiber lines are interfaced with a coaxial cable distribution network that distributes the signals transmitted across the feeder fibers through coaxial cables to a plurality of subscribers.
FIG. 1A illustrates in a simplified manner a full coaxial network architecture. Analog or digital broadband signals are transmitted between a head-end 10 and a multiplicity of coaxial distribution nodes 12, 14, 16, 18, 20, 22, 24, 26 through a coaxial backbone. The coaxial backbone includes various cascaded RF components (not shown) such as line driver amplifiers for example, which are used to boost the signal strength in order to compensate for the attenuation of the signals. At the coax distribution nodes 18, 20, 22, 24, 26, the broadband signal is suitably split off to separate coaxial branches in order to be fed into appropriate coaxial feeder lines to be further distributed to the individual subscribers. In order to increase the number of subscribers served by each coaxial distribution node, coaxial feeder lines associated with the coaxial distribution nodes 18, 20, 22, 24 include multiple RF components 28, 30, 32, 34, 36, 38, 40, 42, 44, and 46.
In the currently operating HFC networks all the coaxial backbones were replaced by fiber optic transmission plants. A fiber plant includes fiber-optic lines, and specific opto-electronic components such as optical transceivers, optical amplifiers, optical switches and the like. The fiber optic plant of the network terminates at specific hybrid fiber-coax distribution nodes, which are coupled to the separate coaxial branches. The branches include coaxial feeder lines and associated RF components. The feeder lines are operative in carrying the broadband signal to the subscribers that are connected to the feeder line via specific connection points referred to typically as taps.
The optical signal carries encoded information units. The signal is transmitted through the fiber plant in the form of light signals at specific wavelengths. At the HFC distribution nodes the optical signals are converted into RF signals and are forwarded via the coaxial branches to the subscribers. With reference to FIG. 1B a highly simplified network of this type of architecture is illustrated. Optical signals are transmitted from a head-end 50 via an optical transceiver 52, a fiber 54, a hub 56, and a fiber 58 to a hybrid fiber-coax distribution node 60. The node 60 includes an optical transceiver and suitable converter units to convert the optical signals into RF signals. The RF signals are transmitted to the subscribers through appropriate coaxial branches, which include coax feeder lines, coax distribution nodes 62, 64, 66, 68 and suitable RF components 70, 72, 74, 76, 78, 80, 82, 84, 86, 88.
The HFC architecture has a number of drawbacks. One disadvantage concerns the relation of the allocated transmission frequency bandwidth for the delivery of the downstream traffic to that of the upstream traffic. As a result of the limited bandwidth of the coaxial plant cable HFC architectures are inherently asymmetrical, with as much as 95 percent of total capacity dedicated to the downstream (head-end to customer) traffic and less than 5 percent is available for upstream (customer to head-end use). In addition, the downstream traffic is a point-to-multipoint architecture while the upstream traffic is multipoint-to-point. As the new electronic services in the market typically provide more interactive capabilities to the subscribers, networks that implement those services require a substantially wider upstream bandwidth to carry the increasingly heavy upstream traffic.
Another disadvantage of the HFC architecture relates to the differences in the transmission capacities of the fiber plant and the coaxial plant of the network. The transmission capacity of the coaxial section is substantially lower than the transmission capacity of the fiber optic section. Thus, the overall transmission capability of the HFC is limited by the transmission characteristics of the coaxial section. This creates a “bottleneck” where the high-capacity fibers are coupled to the substantially lower capacity coaxial cables. As a result of the reduced capacity the coaxial section is capable of serving relatively few subscribers, and thus, requires more feeder fibers to terminate at the hybrid fiber-coax distribution node.
The operators of the presently active HFC systems make every effort to introduce operational and technological improvements into their networks' architecture in order to eliminate, to alleviate, or to reduce the negative effects caused by the above-described drawbacks. A growing list of techniques, both operational and technological, are or will be available to make the flow of traffic within HFC networks more efficient in both directions. One such a group of techniques concerns spectrally efficient modulation methods, referred to typically as “higher order modulations”. For example, replacing the currently prevalent QPSK modulation of the upstream traffic with the spectrally more efficient 16-QAM modulation method may substantially double the total throughput in the upstream direction. Other similar advanced upstream modulations that being proposed or developed are the 256-QAM, the 128-QAM, the 64-QAM, the Advanced PHY, the S-CDMA, and the F-TDMA, and the like.
A completely different technique, which is being accepted for use in the HFC networks is referred to as Physical Node Splitting (PNS). PNS typically halves or quarters the number of homes and businesses sharing the transmission frequency spectrum or certain portions thereof. Most current opto-electric node equipment comes with four output legs where each leg connects to a coaxial section of a network. That yields two potential splits such as from about 500 subscribers to about 250 subscribers, then from about 250 subscribers to about 125 subscribers. Typically, in order to split a node physically, the re-location of the HFC distribution nodes, the addition of HFC nodes, the laying of additional fiber and/or coaxial cables and the installation of additional equipment such as lasers, transmitters, receivers, and the like, are required. The PNS method will be described next in association with FIG. 2.
With reference to FIG. 2 an optical signal is transmitted from the head-end 90 via a fiber 92 to a hub 94. The hub 94 is coupled via the fibers 96, 98, 100, 102 to the HFC distribution nodes 104, 106, 108, 110 respectively. The HFC nodes 104, 108, 110 are coupled to separate branches (not shown) of the coaxial plant. The node 106 is physically split by the installation of additional opto-electric and RF components. The node 106 receives the optical signal, converts the signal to an RF signal and distributes the signal to four respective coaxial branches 109, 111, 107, and 105. The branches 109, 111, 107, and 105 are coaxial cables, which distribute the RF signal to the subscribers associated with the branches. The branch 109 utilizes cascaded RF components 112, 114, 116, 118 to drive the signal to the subscribers. Similarly, the branch 111 utilizes cascaded RF components 128, 130, 132, 134, and 136 to drive the signal to the subscribers, the branch 107 utilizes cascaded RF components 120, 122, 124, 126 to drive the signal to the subscribers, and the branch 105 utilizes cascaded RF components 138, 140, 142, 144, and 146 to drive the signal to the subscribers.
Typically, the location of the operational HFC distribution node does not always identical with the location of the “edge” of the fiber-optic section of the network. Thus, the activities of the Physical Node Splitting usually involve of the laying of additional fiber. The laying of fiber lines demands extensive excavation for the laying of conduit pipes to hold the fiber lines, in addition to the laying of the additional coax lines. In situations where the HFC distribution mode is placed after the “last mile” amplifiers, a change of the amplifier directions as well the change of the direction of passive elements is required. FIG. 3 demonstrates the extension of the fiber plant in association with the physical node splitting method. The HFC distribution node 168 is the end of the original fiber plant that feeds the coaxial branch 150. In addition, the node 168 feeds the optical signal through a fiber 151 to the HFC node 170. The optical signal is transmitted through the fiber 172 to the HFC 174. The HFC 174 is coupled to the coaxial branch 152 via a splitter 176. The HFC 174 converts the optical signal to an RF signal and transmits the signal downstream to the subscribers via the RF components 178, 180, 182, and 184. Upstream signals originated by the subscribers are transmitted upstream via the RF components 198, 196, and 194 having reverse directionality.
Currently, in order to achieve effective physical node splitting the operators are obliged to utilize labor-extensive methods. Typically the fiber nodes should be moved geographically closer to the subscribers' premises, and the coaxial branch should be physically split to additional branches. The process involves complex physical operations such as excavation for the deployment of underground pipes for the placement of the new fiber and the new coax, the re-location or the addition of HFC nodes, and the consequent re-organization of the coaxial branches. Thus, node splitting is a highly complicated process that requires careful planning and organization of the work to be done. As the deployment typically performed in metropolitan areas, it is also highly desirable that the process be completed within a predetermined precise time frame. Consequently the operation involves considerable expenses. Current estimates for physically splitting a single node are in the order of tens of thousands of US dollars. Thus, a key consideration for network design regarding the desirability of physical node splitting is to be able, as much as possible, to match the equipment deployment expense with the expected revenue from the service.
The general objective of the physical node splitting method is to increase the overall transmission capacity of the network. It would be easily perceived to one with ordinary skill in the art that a clear and present need exists for an improved, non-labor-extensive, and cost-effective system and method that could be implemented instead of the physical node splitting method in a hybrid fiber-coax cable network, such as to achieve the same general objective.